The present invention relates to a method for manufacturing an electroluminescence (EL) display.
FIG. 1 shows an AC thin-film EL display of the type having a matrix construction. The EL display has a glass substrate 1, a transparent electrode layer 2, a lower dielectric layer 3, a phosphor layer 4, an upper dielectric layer 5 and a rear electrode layer 6. When voltage is applied to a predetermined position between the transparent electrode layer 2 and the rear electrode layer 6, a strong electric field is formed between the upper dielectric layer 5 and the lower dielectric layer 3 causing electrons to accelerate and collide with particles of the phosphor layer 4 interposed between the layers 5 and 3, thereby emitting light. The emitted light is directed to pass through the transparent electrode layer 2 and the glass substrate 1 to display a predetermined pattern on the substrate 1. The pattern appears on the surface of the substrate 1 in a two dimensional image display due to forming the transparent electrode layer 2 and rear electrode layer 6 perpendicularly in a matrix configuration.
Unfortunately, the EL display shown in FIG. 1 has substantially low image contrast which is a problem for image display, especially in the presence of strong ambient light. The EL display brightness ratio to the ambient light is low, and thus, the image display does not clearly appear on the EL display. Accordingly, the EL display must be used in data processing display devices in which image contrast is not critical.
To solve the contrast problem of the above-mentioned EL display, a black layer 7 is provided between the upper dielectric layer 5 and the rear electrode layer 6 as shown in FIG. 2. The black layer 7 increases the amount of the light emitted to the front, thereby improving the image contrast so that the image display clearly appears on the EL display.
Conventional methods for providing the black layer 7 include the following:
I) Forming a PrMn oxide film by a radio frequency sputtering process.
II) Forming a GaAs film by an electron beam deposition process.
III) Forming a PbTi oxide by thermal decomposition of a dilute organic metal solution.
IV) Using low reflectivity Mo film containing carbon as a rear electrode layer.
V) Forming a carbon film or a black coating applied with black paint by a manual process.
In the above-described methods, the black layer 7 is formed with material different from that of the upper dielectric layer 5 or the rear electrode layer 6. Accordingly, these methods have serious drawbacks.
First, it is difficult to select a material which possesses suitable black layer characteristics, yet does not react with adjacent layers.
Second, providing a black dielectric layer requires an additional fabrication apparatus and additional process steps to separately form a thin film.
Consequently, an EL display with a black layer can not be widely used in practical application because of high production cost; while, on the other hand, an EL display without a black layer has not been used in high resolution display devices because of the low image contrast problem.